Dense rare-earth spin ensembles in solids offer strong collective light-matter coupling for scalable quantum technologies but are typically incompatible with long coherence times due to strong dipolar interactions. Here we challenge this paradigm by showing long-lived coherence in such systems. In a stoichiometric ${\mathrm{EuCl}}_3·6{\mathrm{H}}_2\mathrm{O}$ crystal, we achieve hyperfine coherence times $T_2^{\mathrm{CPMG}}$ of up to 15 s at 4 K, extending the zero-field value $T_2$ of 1.28 ms by more than four orders of magnitude using dynamical decoupling at a zero-first-order Zeeman (ZEFOZ) transition. Homogeneous linewidth measurements reveal a substantial suppression of resonant ${\mathrm{Eu}}^{3+}-{\mathrm{Eu}}^{3+}$ spin interactions introduced by the full concentration near the ZEFOZ point. Coherence remains resilient to strong local perturbations, with a 32 ms coherence time for ${\mathrm{Eu}}^{3+}$ ions adjacent to ${\mathrm{Er}}^{3+}$ dopants. These results establish stoichiometric rare-earth crystals as a promising platform for quantum memories and spin-photon interfaces, demonstrating that intrinsically high optical depth and long-lived spin coherence can coexist in a fully concentrated ensemble.

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• Quantum coherence & coherence measures
• Quantum communication, protocols & technology
• Rare-earth doped crystals